The purpose of this project is to define neural reactions of cat vestibular first order and brain stem neurons in response to angular accelerations delivered in planes precisely oriented to given semicircular canals. In the first two years of the three year grant, we have recorded useful data from 119 horizontal, 50 posterior and 63 anterior canal primary afferents. Initially the head was placed 21 degrees pitch nose down and 6 degrees roll right ear down so the right horizontal canal lay in the plane of horizontal angular acceleration. The starting position for the anterior and posterior canal units was to place these canals precisely in the plane of the turntable. Information was then obtained on resting rate and regularity of firing. Angular accelerations were delivered in the plane of a given canal and then as the canal was positioned at an increasing angle o the turntable. The "null" point, the point at which the sign or polarity of the response reversed as the canal was tipped more and more out of the stimulus plane, was also determined. Another significant finding is that about 25 percent of primary afferents from all the canals had a definite and predictable change in resting discharge with respect to gravity. Precise determination of the planer relations between vestibular receptor activity, brain stem neurons, and ultimately eye movements and direction of vertigo should be useful in determining the active (or hyperactive) receptor in Meniere's disease and space flight.